【转帖】whos responsible for understanding gdt besides the drafter

yang686526

who's responsible for understanding gd&t besides the drafter
drafters and designers are expected to have more than a basic understanding of gd&t, they're expected to have a working knowledge on how to apply it in order to produce fit and function at a reasonable cost.  in addition to the basic courses, many pursue supplemental classes that teach advanced techniques that relate to specific problem solving that may have a direct impact on their design projects.  in my years of working in the automotive and aerospace (spacecraft) industries i've yet to see another person outside our profession hold the same expertise we do.  sure, some inspectors have the basic understanding of single and multiple datum reference frames, or even understand the basic concept of datum precedence but are they expected to have the same level of expertise we have?  what about the machinist?  how many have taken advanced courses and maintained current in asme or iso standards?   my brother is an estimator at a contract shop and confessed to me yesterday that most drawings that come in with a tolerance spec of .002 or under, are simply assigned to their best machine (the hurco)that will insure compliance and produce the whole part better than required.  if the drawing contains anything questionable, they'll call the originator for clarification. they will do whatever it takes to get the part out within specs on time but, they won't spend the extra time to find the virtual conditon of features if they have to take into account multiple variables such as datum shift thru size modifiers on datum references.  time is money and if the drawing requires a lot of it they lose profit.  so to prevent losses, they'll simply run the part on their best machine....even if it means opening up a new shift. do the inspectors have the same working knowledge we do?  i may be wrong in thinking this way but are we wasting our time with calling out more than the basic gd&t needed to define our parts if we can simply add a note on the face of the drawing specifing to be made on their best machine or an overall tolerance their best machine is capable of maintaining since it that's what's likely to occur anyways?   
in my own experience, there should be considerable give and take between the designer, the machinist and the inspector.  they should all have a solid working knowledge of the standard and understand what is trying to be achieved.  i realize that this is not the norm, but i feel that it is necessary if you want to control your design, especially for mass produced parts.
engineers, on the other hand, seem to only know the basics, if that ;)
one of the better companies that i worked for made sure that all effected personel received training in asme y14.5, even the engineers.
i agree with ewh.
anyone that works with the drawings, should have a understanding of gd&t. but, it's not real world. a lot of companies leave it off because they just don't care or understand it.
chris
systems analyst, i.s.
solidworks/pdmworks 05
autocad 05
i have had a lot of exposure to gd&t in my ten years as a mechancial engineer (degreed).  they didn't really go into depth in my degree so i bought a spec and took a class at a local community college.  then my company sent me to two other advanced training classes.  as an engineer i have more of an understanding of how to apply gd&t then our drafters/designers.
i have talked with machine shops that don't have a clue about how gd&t can actually save them time, money and materials.  and don't get me started about functional datums....  i think everyone in the design and manufacturing process should have an understanding of how to apply gd&t and what it means.  to me i consider it a language and a means of communication.
best regards,
heckler
sr. mechanical engineer
sw2005 sp 5.0 & pro/e 2001
dell precision 370
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"coming together is a beginning, staying together is progress, and working together is success." - henry ford

heckler,
those in the engineering, design, and drafting fields as a collective tend to further the advancement and understanding of gd&t by sharing ideas and techniques in an open forum such as this one.  in addition, companies such as yours will pay for additional education in hopes that what was learned in class will eventually trickle down to enhance the overall quality of the product...if not, at least reduce the cost and/or ease the manufacturability of the product.  the roi is worth it to some, if not most companies.  as time goes on, those in our field become experts in communicating thru the use of "ansi speak".  we use this language to communicate on paper our design intent.  the question is, if we're the ones fast pitching the good word, who's catching it at the other end?  do the people (machinist and inspectors) speak it as well as we do....or is the reality of it is that they speak "broken ansi"?   how many of them have you encountered in your advanced courses?   i have only met one inspector in my 24 years proficient enough to know what rules to break.  just pondering the reality of the catchers out there in our industry.
who's responsible?  anyone putting a signature on a document saying that they understand the drawing.  purchasing... sales... qc... tool vendors...
i could be the world's greatest underachiever, if i could just learn to apply myself.
in two places i worked at, gdt was more of a fad and the designers really got carried away with it, trying to put it on almost any type of dimension. i suppose you could put it on an o-ring if you wanted to. trouble was, no one in the shop understood it and most of the designers didn't either. a design supervisor told me about old parts that were updated with gdt, and suddenly the cost skyrocketed. same part, same supplier, same quality (it fit, that is) but now they had to make fixtures to verify the dimensions.
in my case i only have a limited knowledge of it and today you have to be the engineer/designer/drafter all in one. you have to choose what things you can be an expert in. i don't put it on a part unless i absolutely have to. however, i like it for runouts and stuff related to rotating parts because you can pick functional datums and the callouts are easier to use than saying "must be square with the diameter." i try to use plus/minus tolerances instead of true position most of the time- the people in the shop understand it and can measure it.
cost will "skyrocket" if gd&t is not used correctly ... and/or using cheap shops that don't know it to make your parts will raise cost's because it becomes more complicated to them ... or they farm it out becoming the middle man (usually you will never know).
chris
systems analyst, i.s.
solidworks/pdmworks 05
autocad 05
my first class in gd&t was in ~1967, when i was working in the auto industry.  it helped make an e size drawing of an axle shaft more understandable, replacing a huge amount of bulky, awkward text with a few symbols.
thereafter, i used feature control symbols to clarify drawings, by replacing awkward text.. etc.  but i had to be careful about it, because the skill of reading the symbols was not widespread.
it damn near killed off a medical electronics company that i worked for.  it was implemented by stupid zealots.  example; a flat plate 'located' by 13 clearance holes.  worked just fine, until somebody ran the equations, and the plate ended up with ~.255 clearance holes around #10 screws.  then they needed a fixture to locate the plate, because the screws let it wobble around so much that it didn't align with anything.  and they had to put oversize washers under the heads, so the heads wouldn't go right through the clearance holes.
in many cases, it's like giving a child a chainsaw.
  
mike halloran
pembroke pines, fl, usa
i designed a part a few years that was measured in microns. it was about the size of a pin head. it had to be machined, not stamped. the drawing was d-size and the scale was about 100/1. the dims & tolerances were about 4-6 dec places and had to mate with the customer's part. i had gd&t all over. after purchasing, vendor, mngmt, inspection and engineer all started using the drawing, it came back to me marked up with dims/tol changed to 3 dec places and gd&t removed. they were all looking at the huge view of the part as if it were a bigger part. a couple thousand parts were scrapped.
if they listened to the designer, it would not have had happened.
chris
systems analyst, i.s.
solidworks/pdmworks 05
autocad 05
i worked on a navy base about 15 years ago. (the boss was such a sphincter that my memories are still vivid--i'm surprised it was 15 years ago because it seems like only a few. but that is for another thread in another forum...) the decision was made long before i got there that all tolerancing--where applicable--would be in gd&t. i got used to using diametral position of .084" (± .030"). it was smooth as glass. checking,  inspection, design was all easy because determining fit was reduced (in most cases) to arithmetic instead of algebra. they also disproved the axioms i hear all the time that it raises the price of machining and that it only needs to be used when absolutely necessary. we spent no more on fabrication than i had at other places. we just made sure we had shops that were not only competent in fabrication, but in gd&t. and most of the engineering/design/drafting personnel were well trained.
gd&t was used as it was intended--as a method of locating and controlling features that allows more parts to be accepted.
very sweet.
i have to confess that i have very little patience anymore with shops that say they don't understand gd&t nearly 40 years after it became a national standard. my response is usually, "find a shop that's current. do they still use dial mills too?" precision tolerancing and gauging is only 108 years old. gd&t first hit the scene in the uk, before wwii! how long does it take for a competent understanding to take hold in the industry?